Traditional methods of preparing ceramic articles do not readily lend themselves to the preparation of ceramic matrix composite materials, especially fiber- and/or wire-reinforced ceramic matrix composite structures. A composite structure is one which comprises two or more different materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite structure typically comprises a ceramic matrix which embeds one or more diverse kinds of filler materials such as particulates, fibers, rods or the like.
Traditional methods of preparing ceramic articles involve the following general steps: (1) preparation of material in powder form; (2) grinding or milling of powders to obtain very fine particles; (3) formation of the powders into a body having the desired geometry (with allowance for shrinkage during subsequent processing), e.g., this step might be accomplished by uniaxial pressing, isostatic pressing, injection molding, tape casting, slip casting or any of several other techniques; (4) densification of the body by heating it to an elevated temperature such that the individual powder particles merge together to form a coherent structure, e.g., this step is preferably accomplished without the application of pressure (i.e., by sintering), although in some cases an additional driving force is required and can be provided through the application of external pressure either uniaxially (e.g., hot pressing) or isostatically (e.g., hot isostatic pressing); and (5) finishing, frequently by diamond grinding, as required.
In the preparation of ceramic matrix composite materials, the most serious difficulties with traditional methods arise in the densification step, number (4) above. The normally preferred method, pressureless sintering, can be difficult or impossible with particulate composites if the materials are not highly compatible. More importantly, normal sintering is impossible in most cases involving fiber composites even when the materials are compatible, because the merging together of the particles is inhibited by the fibers which tend to prevent the necessary displacements of the densifying powder particles. These difficulties have been, in some cases, partially overcome by forcing the densification process through the application of external pressure at high temperature. However, such procedures can generate many problems, including breaking or damaging of the reinforcing fibers by the external forces applied, limited capability to produce complex shapes (especially in the case of uniaxial hot pressing), and generally high costs resulting from low process productivity and the extensive finishing operations sometimes required.
Additional difficulties can also arise in the body formation step, number (3) above, if it is desired to maintain a particular distribution of the composite second phase within the matrix. For example, in the preparation of a fibrous ceramic matrix composite, the powder and fiber flow processes involved in the formation of the body can result in non-uniformities and undesired orientations of the reinforcing fibers, with a consequent loss in performance characteristics.
Other methods are also used as means for forming ceramic matrix composites. For example, the formation of a matrix structure by the reaction of gaseous species to form the desired ceramic (a process known as chemical vapor deposition) is employed currently for silicon carbide fiber-reinforced silicon carbide matrix composites. This method has met with only limited success, partly because the matrix deposition process tends to occur on all of the composite second phase surfaces at once, such that matrix development only occurs until the growing surfaces intersect, with the trapping of porosity within the body being an almost inevitable consequence. In addition, the rate of matrix deposition has been so low as to make such composites prohibitively expensive for all but the most esoteric applications.
A second non-traditional approach involves the infiltration of the composite particles or fibers with a flowable organic material containing the necessary elements to form the desired ceramic matrix. Ceramic formation occurs by chemical reaction on heating this material to an elevated temperature. Once again, limited success has been achieved, in this case because elimination of the large amounts of volatile materials (necessary constituents of the initial flowable infiltrant composition) during the heating process tends to leave behind a porous and/or cracked ceramic body.
Talsma (U.S. Pat. No. 3,255,027) discloses forming a particulate mixture comprising up to 81 weight percent filler, an aluminum alloy particulate and at least about 0.02 weight percent of the aluminum alloy as a fluxing agent which is preferably a metal oxide or hydroxide. The particulate mixture is fired in an oxygen-containing atmosphere to a temperature of at least 600.degree. C. for a time sufficient to convert at least 11 percent of the aluminum to aluminum oxide. The formed body comprises 15-95 percent by volume porosity, less than 81 percent by weight of filler, less than 81 percent by weight of residual aluminum alloy and at least 19 percent by weight of aluminum oxide. However, Talsma teaches away from the use of a nitrogen-containing atmosphere.
Bechtold (U.S. Pat. No. 3,262,763) discloses refractory bodies produced by heating compacts which comprise by weight about 16-70 percent aluminum, 15-85 percent silicon nitride, 1-45 percent boron in a nitrogen or oxygen-containing atmosphere to a temperature between about 700.degree. C. and about 1500.degree. C. The bodies so formed contain some porosity and at least about 5 percent excess aluminum or silicon. The bodies may be electrically conductive or insulating. However, Bethtold does not teach the use of a substantially inert filler material to serve as a reinforcement phase for the body to be produced.
Bawa (U.S. Pat. No. 3,421,863) discloses a method for making a cermet material which is electrically insulating at elevated temperatures. The method comprises compression molding a particulate mass comprising 80-98 percent by weight of aluminum powder and 2-20 percent of aluminum silicate powder (such as Kaolin clay) and firing in an oxygen-containing atmosphere for about 4-8 hours at a temperature between about 1000.degree. C. and about 1400.degree. C. Bawa does not disclose the use of a nitrogenous atmosphere to produce an aluminum nitride-based ceramic body.
Seufert (U.S. Pat. No. 3,437,468) discloses certain composite materials made by a reaction process with molten aluminum. However, the matrix constituent of these materials inherently contains a large amount of magnesium aluminate spinel, a material of less desirable properties (for example, lower hardness) than certain other ceramics such as aluminum oxide. In addition, the process of the Seufert Patent requires that the ceramics be formed, in major part, by reaction of aluminum with magnesium oxide and silicon dioxide (in free or combined form) which reduces the flexibility of the process and dictates that substantial amounts of silicon (in addition to magnesium aluminate) will be present in the matrix of the final ceramic product.
Oberlin (U.S. Pat. No. 3,473,938) discloses a method for producing an aluminum oxide-based honeycomb structure by coating an aluminum honeycomb structure with a composition comprising 2-25 percent by weight of vanadium oxide (B.sub.2 O.sub.5), 10-98 percent of a fluxing agent comprising alkali metal silicates and alkaline earth silicates, and 0-90 percent of one or more of the following: Al.sub.2 O.sub.3, MgO, Cr.sub.2 O.sub.3, TiO.sub.2, ZrSiO.sub.4, MgSiO.sub.3, M-silicate and Al. The coated aluminum honeycomb structure is then heated in an oxygen atmosphere for at least 8 hours at a temperature between about 600.degree. C. and about 900.degree. C., then at least 10 hours at a temperature between about 900.degree. C. and the melting point of the formed body. Again, Oberlin does not disclose the use of a nitrogenous atmosphere to produce aluminum nitride. Further, Oberlin uses a single body of metal and not a plurality of finely divided parent metal bodies.